1. Field of the Invention
The present invention relates to a process for production of an oriented electrical steel sheet, and more particularly to an oriented electrical steel sheet having a low iron loss particularly by using a technique for heating a slab at a low temperature.
2. Description of the Prior Art
An oriented electrical steel sheet is used mainly as an iron core material for a transformer, a generator and other electrical equipment and should have excellent magnetic properties, particularly iron loss properties.
An oriented electrical steel sheet is produced by developing a crystal grain having the so-called "Goss orientation", that is, having a (110) face on the rolled surface and an [001] axis in the rolling direction by utilizing a secondary recrystallization phenomenon.
As is well known in the art, secondary recrystallization occurs in finish annealing. In this case, the so-called "inhibitor", which is a fine precipitate of AlN, MnS, MnSe or the like for regulating the growth of a primary recrystallized grain until the temperature reaches a secondary recrystallization region, should be present.
For this reason, an electric steel slab is heated to a high temperature, for example, about 1350.degree. to 1400.degree. C. so as to form an inhibitor, for example, AlN, MnS or MnSe, in a solid solution, and annealing is performed for finely precipitating the inhibitor when the material is in the form of a hot rolled sheet or an intermediate sheet before final cold rolling.
Such a treatment has enabled an oriented electrical steel sheet having a high magnetic flux density to be produced. Since, however, the electrical steel slab is heated at the above-described high temperature, there occurs a large amount of a molten scale, which hinders the operation of a heating furnace. Further, this process has problems in that it requires a high energy unit and it creates surface defects.
For this reason, studies have been made on a process for producing an oriented electrical steel sheet at a lowered slab heating temperature. For example, Japanese Unexamined Patent Publication (Kokai) No. 55-24116 discloses a process wherein a slab is heated at 1100.degree. to 1260.degree. C. through the incorporation of a nitride forming element, such as Zr, Ti, B, Ta, V, Cr or Mo, in addition to Al. Further, Japanese Unexamined Patent Publication (Kokai) No. 59-56522 proposes a process wherein an electrical steel slab having a Mn content of 0.08 to 0.45%, a S content of 0.007%, a lowered content [Mn].times.[S] and further comprising Al, P and N is used as the material.
The process wherein the slab is heated at a low temperature exhibits certain functions and effects. In this process, however, since the inhibitor forming ingredient, for example, Al, Mn, S, Se or N, is not completely dissolved in the steel, the formation of an inhibitor useful for the development of a secondary recrystallization is the task of this process.
In Japanese Unexamined Patent Publication (Kokai) No. 2-200732, the applicant for the present invention has proposed a process wherein when an oriented electrical steel sheet cold-rolled to a predetermined sheet thickness is passed in the form of a strip, through a decarburization annealing furnace, and the sheet is nitrided by using NH.sub.3 to form an inhibitor in situ.
In a process for producing an oriented electrical steel sheet that comprises nitriding a steel sheet subjected to decarburization annealing by using a gas having a nitriding capability to strengthen the inhibitor, coating the nitrided sheet with an annealing separator composed mainly of MgO, taking up the coated sheet in coil form and subjecting the sheet to finish annealing; the development of the secondary recrystallization varies despite an identical degree of nitriding, which often gives rise to a variation in the magnetic flux density and iron loss or an inferior secondary recrystallized grain called "fine grain".